Optical sensor for surface inspection and metrology

ABSTRACT

An optical system configured to measure a raised or receded surface feature on a surface of a sample may comprise a broadband light source; a tunable filter configured to filter broadband light emitted from the broadband light source and to generate a first light beam at a selected wavelength; a linewidth control element configured to receive the first light beam and to generate a second light beam having a predefined linewidth and a predetermined coherence length; collimating optics optically coupled to the second light beam and configured to collimate the second light beam; collinearizing optics optically coupled to the collimating optics and configured to align the collimated second light beam onto the raised or receded surface feature of the sample, and a processor system and at least one digital imager configured to measure a height of the raised surface or depth of the receded surface from light reflected at least from those surfaces.

FIELD OF THE INVENTION

The embodiments discussed in this disclosure are related to an imagerand imaging interferometer.

BACKGROUND OF THE INVENTION

An interferometer utilizes superimposed waves, such as visible light orelectromagnetic waves from other spectral regions, to extractinformation about the state of the superimposed waves. Thesuperimposition of two or more waves with the same frequency may combineand add coherently. The resulting wave from the combination of the twoor more waves may be determined by the phase difference between them.For example, waves that are in-phase may undergo constructiveinterference while waves that are out-of-phase may undergo destructiveinterference. The information extracted from the coherently added wavesmay be used to determine information about a structure that interactswith the waves. For example, interferometers may be used for measurementof small displacements, refractive index changes, and surfacetopography. In imaging interferometer, interferometric measurements overa finite area of the surface, instead of a point on the surface, isachieved by using imaging sensors. An imager utilizes reflected andscattered waves, such as visible light or electromagnetic waves fromother spectral regions, to extract information about the state,location, and topology of surface defects.

The subject matter claimed in this disclosure is not limited toembodiments that solve any disadvantages or that operate only inenvironments such as those described above. Rather, this background isonly provided to illustrate one example technology area where someembodiments described in this disclosure may be practiced.

SUMMARY OF THE INVENTION

According to an aspect of one or more embodiments, an interferometer mayinclude tunable light sources, splitters, digital imagers, and processorsystems. The tunable light source may be configured to emit a lightbeam. The splitters may be configured to direct the beam toward a samplewith a reference plate, a raised surface feature and a feature floorsurface. According to an embodiment, a first digital imager may beconfigured to receive a reflected beam to generate an interferencepattern and to generate an image based on the reflected beams. Thereflected beam may be a coherent addition of a first reflection of thebeam off the surface of the reference plate, a second reflection of thebeam off the raised surface feature and a third reflection off the floorof the sample or substrate on which features are located. The processorsystem may be coupled to the digital imager and may be configured todetermine a distance between the reference surface and the raisedsurface features based on the generated image or images.

According to an aspect of one or more embodiments, methods ofdetermining the thickness of a sample are also disclosed. The methodsmay include emitting a light beam and directing the light beam toward asample with a floor surface and a feature surface. The methods may alsoinclude generating an image based on a reflected light beam. Thereflected light beam may be a coherent addition of a first reflection ofthe light beam off the reference plate, a second reflection of the lightbeam off of the top surface of the sample and a third reflection of thelight beam off of the bottom surface of the sample. The method may alsoinclude determining a thickness between the top surface and the bottomsurface based on the reflections.

A second digital imager may be configured to receive reflected andscattered beams to generate an image of protruding features and thesample floor on which the features are positioned. The processor systemmay be coupled to the second digital imager and may be configured tobuild a two-dimensional image of the raised surface features and thefloor of the sample.

One embodiment is a method, comprising emitting a first light beam of afirst wavelength at a first time; directing the first light beam towarda reference plate and a first sample facing the reference plate, thefirst sample defining a floor surface and a raised surface featurerising above the floor surface; generating a first image based on afirst reflected light beam, the first reflected light beam being acoherent addition of a first reflection of the first light beam off ofthe reference plate, a second reflection of the first light beam off ofthe raised surface feature and a third reflection of the first lightbeam off of the floor surface, and based on the generated first image,determining a distance between the surface of the reference plate andthe raised surface feature and determining a distance between thesurface of the reference plate and the floor surface.

According to further embodiments, the method may further compriseemitting a second light beam of a second wavelength at a second timedifferent from the first time; directing the second light beam towardthe first sample; generating a second image based on a reflected secondlight beam, the reflected second light beam being a coherent addition ofa first reflection of the second light beam off of the reference plate,a second reflection of the second light beam off of the raised surfacefeature and a third reflection of the second light beam off of the floorsurface, and based on the generated first and second images, determiningthe distance between the surface of the reference plate and the raisedsurface feature, and a distance between the reference plate and thefloor surface.

The method may further comprise selecting a wavelength differencebetween the first wavelength and the second wavelength based on adistance between the floor surface and the raised surface feature; andselecting a first bandwidth of the first light beam and a secondbandwidth of the second light beam based on a distance between thesurface of the reference plate and different raised surface features onthe first sample. According to one embodiment, determining the distancebetween the reference plate and the raised surface feature based on thefirst image and the second image may include constructing a fringepattern based on first intensity values of pixels from the first imageof and second intensity values from pixels of the second image; andperforming a frequency domain transform on the constructed fringepattern. For instance, the frequency domain transform may be a FastFourier Transform, Discrete Fourier Transform, a Discrete CosineTransform or a Hilbert Transform.

In one embodiment, determining the distance between the reference plateand the raised surface feature based on the first image may comprisecomparing an intensity of a pixel of the image to a plurality ofmodel-based pixel intensities that correspond with respective differentdistances; and selecting one of the plurality of model-based pixelintensities that is closest to the intensity of the pixel, wherein thedetermined distance is the distance corresponding to the selected one ofthe plurality of model-based pixel intensities.

According to one embodiment, the first sample is part of a semiconductorformed on a wafer, and the method may further comprise emitting a secondlight beam; directing the second light beam toward a second sample ofthe semiconductor, the second sample being unilluminated by the firstlight beam and located on a different part of the semiconductor than thefirst sample; generating a second image based on a reflected secondlight beam, the reflected second light beam being a coherent addition ofa first reflection of the second light beam off of the reference plate,of a second reflection of the second light beam off of a raised surfacefeature on the second sample and a third reflection of the second lightbeam off of the floor surface of the second sample; and based on thesecond image, determining second distances between the reference plateand the raised surface feature of the second sample, and between thereference plate and a floor surface of the second sample.

In one embodiment, an auto-correlation interferometer function may becarried out on at least the first reflected light beam.

According to some embodiments, directing the first light beam maycomprise directing the first light beam toward the reference plate andthe first sample at an angle normal or substantially normal to thereference plate. The method may also comprise a second light beam;directing the second light beam toward the reference plate and the firstsample at angles away from normal to the floor surface of the sample;generating a second image based on a second reflected light beam, thesecond reflected light beam being a coherent addition of a secondreflection/scatter of the second light beam off of the reference plate,a second reflection/scatter of the second light beam off of the raisedsurface feature and a third reflection/scatter of the second light beamoff of the floor surface; and based on the generated second image,determining the distance between the reference plate and the raisedsurface feature and determining the distance between the reference plateand the floor surface.

Another embodiment is an optical system configured to measure raisedsurface features on a surface of a sample. The optical system maycomprise a first digital imager and a second digital imager; a processorsystem coupled to the first and to the second digital imagers; a tunablelight source configured to emit a first light beam and a second lightbeam; a first lens system (comprising one or more lenses) configured tofocus incident light onto the sample surface and to focus reflectedlight back on the first and second digital imagers; a reference platedisposed above and facing the sample surface; an off-axis ringilluminator and configured to receive and direct the second light beamtoward the reference plate at angles other than normal to the samplesurface; a first beam splitter configured to direct the first light beamthrough the first lens system toward the reference plate in a directionnormal to the sample surface and to transmit a first reflected lightbeam of the first light beam reflected by the reference plate and asecond reflected light beam of the second light beam reflected by the araised surface feature on the sample; a second beam splitter configuredto transmit at least a portion of the first reflected light beam towardthe first digital imager and to reflect at least a portion of the secondreflected light beam toward the second digital imager; wherein the firstdigital imager is configured to generate a first digital image based atleast on the first reflected light beam and wherein the second digitalimager is configured to generate a second digital image based at leaston the second reflected light beam, and wherein the processor system isconfigured to determine a distance between the reference plate and theraised surface feature of the sample based at least on the first andsecond digital images

The first reflected light may comprise a coherent addition ofreflections of the first light beam on the reference plate, on theraised surface feature and on the sample surface. Similarly, the secondreflected light may comprise a coherent addition of reflections of thesecond light beam on the reference plate, on the raised surface featureand on the sample surface.

In one embodiment, the off-axis ring illuminator is disposed co-axiallywith the first lens system.

The processor system may be further configured to generate an image ofthe sample surface for defect inspection and to generate a fluorescenceimage of the sample for residue-defect inspection. According to someembodiments, the second beam splitter may include a dichroic mirrorconfigured to primarily transmit a first pre-determined band ofwavelengths and to primarily reflect a second pre-determined band ofwavelengths, wherein the first light beam has a wavelength within thefirst pre-determined band of wavelengths and wherein the second lightbeam has a wavelength within the second pre-determined band ofwavelengths.

According to an embodiment, the first digital imager and the seconddigital imager may be configured to generate digital images based onsample fluorescence emission, generated via excitation of the samplesurface and of the raised surface feature by the first and second lightbeams.

The first and/or the second digital imagers may comprise, for example, atwo-dimensional array of CCD or CMOS pixels or a one-dimensional linearray of CCD or CMOS pixels having pixel readout rate ranging from tensof Hz to hundreds of kHz, such as from 10 Hz to 300 KHz.

The reference plate positioned above the sample surface may have anoptical thickness configured to provide a near common path Mirauinterferometer functionality.

The optical system may be further configured to optically scan thesample surface using a translation system whose travel speed across afield of view of the optical system determines a sampling pixel size ofthe first and second images. The translation system may be configured tomove the sample across the field of view of the optical system and/or tomove the optical system across the sample.

According to some embodiments, the first light beam has a firstwavelength and is emitted at a first time, and the second light beam hasa second wavelength that is different form the first wavelength and isemitted at a second time that is different than the first time. Theprocessor system may be configured to determine the distance between thereference plate and the raised surface feature based on the firstdigital image and the second digital image. The tunable light source maybe configured to emit a plurality of light beams, the plurality of lightbeams including the first light beam and the second light beam, each ofthe plurality of light beams having a different wavelength, a number ofthe plurality of light beams being selected based on the distancebetween the reference plate surface and the raised surface feature. Thedetermined distance between the reference plate surface and the raisedsurface feature may then be based on a plurality of first and secondimages generated by the first and second digital imagers based on theplurality of light beams. The tunable light source may comprise abroadband light source configured to emit the first light beam at afirst time and the second light beam at a second time; and a tunablefilter that is configured to filter the first light beam to have a firstwavelength and to filter the second light beam to have a secondwavelength.

According to one embodiment, the distance between a reference surface onthe sample or the reference plate and the raised surface feature may bedetermined based on a first intensity value of a first pixel location inthe first digital image and a second intensity value of a correspondingfirst pixel location in the second digital image.

The first lens system may be positioned between the sample and the firstbeam splitter, and the optical system may further comprise a second lenssystem positioned between the first and the second beam splitters; andan adjustable system aperture positioned one in an aperture plane of thefirst lens system, or between the first lens system and the second lenssystem. The size of an aperture of the adjustable system aperture may beconfigured to be adjustable based on a lateral resolution and/or a fieldof view of the optical system.

In one embodiment, the first beam splitter may be movably controllableto redirect the first light beam away from the raised surface featuretoward an other part of the sample, such that the raised surface featureis not illuminated (unilluminated) by the redirected first light beamand the processor system may be configured to determine a distancebetween the reference plate and the other part of the sample.

According to some embodiments, the second digital imager may beconfigured to receive at least a portion of the first light beam thatincludes a reflected light beam from normal incidence illumination andat least a portion of the second light beam that includes scatteredlight beam from off-axis illumination to generate at least one thirddigital image based on the reflected and scattered light beams and theprocessor system may be further configured to generate a two-dimensionaldigital image from the at least one third digital image.

In one embodiment, the sample may be moved under the imaging system by ax-y translation system comprising a moving stage and wherein the seconddigital imager may be configured to receive at least a portion of thefirst light beam that includes a reflected light beam from normalincidence and at least a portion of the second light beam that includesscattered light beam from off-axis illumination to generate thirddigital image(s) based on the reflected and scattered light beams. Theprocessor system may be further configured to generate a two-dimensionaldigital image of an entire surface of the sample from the third digitalimage(s).

According to some embodiments, the tunable light source may beconfigured to emit a single wavelength of light beam toward the sampleand to enable repeated multiple single wavelength inspection of thesample in both a bright field mode and in a dark field mode. The singlewavelength of light beam incident on the sample surface may have awavelength that causes the sample surface to fluoresce, and the seconddigital imager may be configured to receive a fluorescence emission fromthe sample surface via a first dichroic mirror as the sample is movedunder the imaging system by translation system comprising a movingstage; and the second processor system may be configured to generate atwo-dimensional digital image based on the received fluorescenceemission.

The optical system may further comprise an auto-correlationinterferometer comprising a first movable mirror and a second movablemirror both disposed along an optical path of the second reflected lightbeam and aligned with the second digital imager.

According to another embodiment, an optical system configured to measurea raised or receded surface feature on a surface of a sample maycomprise a broadband light source; a tunable filter optically coupled tothe broadband light source, the tunable filter being configured tofilter broadband light emitted from the broadband light source and togenerate a first light beam at a selected wavelength; a linewidthcontrol element configured to receive the first light beam and togenerate a second light beam having a predefined linewidth and acoherence length that is a function of a minimum height of the raised orreceded surface feature on the sample; collimating optics opticallycoupled to the second light beam and configured to collimate the secondlight beam; collinearizing optics optically coupled to the collimatingoptics and configured to align the collimated second light beam onto theraised or receded surface feature of the sample, and a processor systemand at least one digital imager configured to measure a height of theraised or receded surface from light reflected at least from the raisedor receded surface.

According to further embodiments, the broadband light source maycomprise a white light laser and/or an ultra-broadband source. Theultra-broadband source may comprise an arc lamp, a laser driven plasmasource and/or a super luminescent diode (SLED). The tunable filter maybe configured to generate different wavelengths of light having a finitelinewidth at each wavelength. The broadband light source and the tunablefilter may form a tunable light source that may be configured togenerate the first light beam at a plurality of wavelength steps andsuch that the first light beam has a predetermined linewidth at each ofthe plurality of wavelength steps. The linewidth control element maycomprise a plurality of interference filters, each of the plurality ofinterference filters having a defined passband. The linewidth controlelement may comprise a grating-based wavelength selector element. Thelinewidth control element may be configured to receive a controlled beamdiameter of the first light beam. The optical system may furthercomprise a variable beam expander disposed between the tunable filterand the grating-based wavelength selector element, the variable beamexpander being configured to selectively control a beam diameter of thefirst light beam incident upon the grating-based wavelength linewidthselector element. The variable beam expander may be configured to tunethe coherence length of the second light beam for a given pitch anddiffraction order of the grating-based wavelength selector element atany wavelength of the first light beam. The optical system may furtherbe configured to image differing z-height ranges of a plurality ofraised or receded surface features on a same or a different sample,through control of wavelengths of the first light beam by the tunablefilter and through control, by the variable beam expander, of the beamdiameter incident on the grating-based wavelength selector element. Thetunable filter may comprise an acousto-optic tunable filter (AOTF).

Another embodiment is a method of measuring a height of a raised orreceded surface feature on a surface of a sample. The method maycomprise emitting a broadband light beam; selectively filtering thebroadband light beam to generate a first light beam at a selectedwavelength; controlling a linewidth of the first light beam to generatea second light beam having a predefined linewidth and a coherence lengththat is a function of a minimum height of the raised or receded surfacefeature on the sample; collimating the second light beam; collinearizingthe collimated second light beam and directing the collinearized secondlight beam onto the raised or receded surface feature of the sample, andmeasuring the height of the raised or receded surface feature on asurface of a sample based upon light reflected at least from the raisedor receded surface feature.

According to further embodiments, emitting may be carried out using awhite light laser and/or an ultra-broadband source. The ultra-broadbandsource may comprise an arc lamp, a laser driven plasma source and/or asuper luminescent diode (SLED). Selectively filtering may be carried outusing a tunable filter configured to generate different wavelengths oflight having a finite linewidth at each wavelength. Emitting andselectively filtering, according to an embodiment, may be carried outsuch that the first light beam has a predetermined linewidth at each ofa plurality of wavelength steps. Controlling the linewidth of the firstlight beam may be carried out by passing the first light beamselectively through one of a plurality of interference filters, each ofthe plurality of interference filters having a defined passband.Controlling the linewidth of the first light beam may be carried outusing a grating-based wavelength selector element. The method mayfurther comprise selectively controlling a beam diameter of the firstlight beam incident upon the grating-based wavelength selector element.The method may further comprise tuning, by variable beam expander, thecoherence length of the second light beam for a given pitch anddiffraction order of the grating-based wavelength selector element atany wavelength of the first light beam. The method may further compriseimaging differing z-height ranges of a plurality of raised or recededsurface features on a same or a different sample by successivelycontrolling wavelengths of the first light beam, and controlling of adiameter of the first light beam incident on the grating-basedwavelength selector element. Selectively filtering may compriseselectively filtering the first light beam according to a frequency ofan acoustic input signal.

A still further embodiment is a method of improving axial resolution ofinterferometric measurements of a raised feature and a floor feature ofa sample. According to one embodiment, the method may compriseilluminating the features of the sample using a first limited number ofsuccessively different wavelengths of light at a time; generating animage of at least the feature based on intensities of light reflectedfrom the feature at each of the successively different wavelengths oflight; measuring a fringe pattern of intensity values for eachcorresponding pixel of the generated images; resampling the measuredfringe patterns as k-space interferograms; estimating interferencefringe patterns for a spectral range that is longer than available fromthe generated images based on the first limited number of successivelydifferent wavelengths of light using the k-space interferograms;appending the estimated interference fringe patterns to the respectivemeasured fringe patterns of intensity values generated from illuminatingthe raised feature of the sample using the first limited number ofsuccessively different wavelengths of light; measuring the height of theraised feature using the measured interference fringe patterns andappended estimated fringe patterns with improved axial resolution tosimulate the effects of illuminating a top and a floor of the raisedfeature of the sample using a second limited number of successivelydifferent wavelengths of light, the second limited number ofsuccessively different wavelengths of light being greater than the firstlimited number of successively different wavelengths of light.

According to further embodiments, estimating the fringe patterns maycomprise modelling the k-space interferograms using an algebraic modelof k-space intensities having a plurality of coefficients and using theplurality of coefficients to generate the estimated fringe patterns. Thealgebraic model of k-space intensities, according to one embodiment, maybe expressed as I(k)=A[1+B·Cos(C)] where, A=I_(max)+I_(min))/2 is a DCamplitude of the plotted fringe pattern,B=(I_(max)−I_(min))/(I_(max)+I_(min)) is a fringe visibility, and whereC=2k·d, is the phase factor at each wavelength for a distance d.Measuring the height of the raised feature may, according to oneembodiment, include applying a discrete frequency domain transformationto the plotted interference fringe pattern and appended estimated fringepatterns. Measuring the height of the raised feature may includeapplying a model-based fringe analysis technique. The discrete frequencydomain transformation may be, for example, a Fast Fourier Transform(FFT) or a Hilbert Transform. Measuring the height of the raised featureusing the plotted interference fringe pattern and appended estimatedfringe patterns may introduce a fixed bias error and the method mayfurther comprise cancelling out the introduced fixed bias error.Illuminating may be carried out by a light source and the method maycomprise moving the sample past the light source at a first speed thatis proportional to first limited number of successively differentwavelengths of light and not at second, slower speed proportional to thesecond limited number of successively different wavelengths of light.The method may further comprise carrying out an iterative process tominimize a least-square criterion between measured interference fringepatterns and the appended estimated fringe patterns.

The object and advantages of the embodiments will be realized andachieved at least by the elements, features, and combinationsparticularly pointed out in the claims. It is to be understood that boththe foregoing general description and the following detailed descriptionare exemplary and explanatory and are not restrictive of the invention,as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1A illustrates an example interferometer system according to anembodiment;

FIG. 1B illustrates multiple interferometer sub-systems according to anembodiment;

FIG. 2 illustrates another example interferometer system according to anembodiment;

FIG. 3 illustrates an example interferometer that includes autocorrelation interferometry according to an embodiment;

FIG. 4 illustrates an example of beam reflection off an examplesemiconductor device according to an embodiment; and

FIG. 5 illustrates an example of beam reflection off another examplesemiconductor device according to an embodiment.

FIG. 6 illustrates tunable coherence length stretching that may be usedin interferometer systems to measure topological features having agreater z-height profiles, according to another embodiment.

FIG. 7 illustrates tunable coherence length stretching with centralwavelength stabilization that may be used in interferometer systems tomeasure topological features having a greater z-height profiles,according to yet another embodiment.

FIG. 8A shows a fringe pattern obtained with 400 wavelengths of light,to illustrate additional aspects of embodiments;

FIG. 8B shows an interference fringe pattern obtained with 21wavelengths of light, to illustrate additional aspects of embodiments;

FIG. 9A shows an interference fringe pattern obtained with 21wavelengths of light and FIG. 9B shows the relatively low axialresolution obtainable therefrom.

FIG. 10A shows an interference fringe pattern obtained using 21wavelengths of light, extended by estimated spectral data to simulate aninterference fringe pattern obtained using a greater number ofwavelengths of light, and FIG. 10B shows the comparably higher axialresolution obtainable from the extended spectral extension of FIG. 10A.

DESCRIPTION OF EMBODIMENTS

According to at least one embodiment, an interferometer system mayinclude a tunable light source, a beam direction unit, an imaging lensassembly, a digital imager, and a processor system. The interferometermay be configured to determine a distance between a reference surface, araised surface feature and floor surface of a sample. The sample may bea portion of a surface of a semiconductor device constructed or formedon a wafer. In some embodiments, the reference surface may be a topsurface of the wafer and the feature surface may be a top surface of thesemiconductor device formed on the wafer. In other embodiments, thereference surface may be a transparent surface not part of the wafer andthe raised surface feature may be a top surface or floor surface of thesemiconductor device built on the wafer.

In some embodiments, the tunable light source may be configured to emita light beam with a first wavelength at a first time. The beam splittermay be configured to direct the beam toward the sample. The beam mayreflect off the sample. In some embodiments, the beam may reflect off areference surface to generate a first reflected beam. The beam may alsoreflect off the raised surface feature to generate a second reflectedbeam and a third reflected beam off of the reference surface, such as areference plate. The first, second and third reflected beams may addtogether coherently to form a reflected imaging beam that is received bythe digital imager. The digital imager may be configured to generate adigital image based on an intensity of the reflected imaging beam.

The tunable light source may be configured to emit multiple other lightbeams, each at a different time. Each of the multiple other light beamsmay have a different wavelength. A digital image may be generated by thedigital imager for each of the multiple other light beams in a similarmanner as the digital image was generated for the light beam with thefirst wavelength.

The processor system may be configured to receive and compare thedigital images from the digital imager or imagers. Based on thecomparison between intensity values at the same pixel locations in thedigital images, the processor system may be configured to determine adistance between the reference surface or reference plate and differentraised surface features and floor surface on the sample.

In some embodiments, the sample may be a single location. Alternately oradditionally, the sample may correspond to an area of the semiconductor.In these and other embodiments, the processor system may be configuredto determine a topology of the sample over the area of the semiconductorbased on the digital images. The topology may represent the distancebetween the reference plate and the floor of the sample and the distancebetween the reference plate and the raised surface features over thearea of the semiconductor substrate.

In some embodiments, the interferometer may include one or more lensesor lens systems and an adjustable system aperture in the imaging path(i.e., the path of the reflected light beams). The lens or lenses may beconfigured to focus the imaging beam on the digital imagers. Theadjustable system aperture may be configured to adjust the spatialresolution of the imaging system. In these and other embodiments, thefield of view of the digital imagers may correspond to the area of thesemiconductor substrate for which the distances between the referenceplate and the raised surface features over the area of the semiconductorsubstrate are to be determined.

In some embodiments, a system may include multiple interferometersystems. In these and other embodiments, each of the systems maydetermine a distance between a reference surface on the sample (or aseparate reference plate) and a feature surface of the semiconductor fordifferent portions of the semiconductor at the or nearly the same time.In this manner, a topology of an entire semiconductor may be morequickly determined than when using a single interferometer.

In some embodiment, the interferometric imaging system may be configuredto include a separate imaging channel that incorporates a second digitalimager configured to generate a digital image based on an intensity ofthe return beam reflected and scattered from the sample surface.

Embodiments of the present disclosure will be explained with referenceto the accompanying drawings.

FIG. 1A illustrates an exemplary system 100 a (the “system 100 a”),arranged in accordance with at least some embodiments described in thisdisclosure. The system 100 a may be configured to image a raised surfacefeature 113 a on sample 112 that is part of, for example, asemiconductor device 130. To generate the sample surface image, thesystem 100 a may include a tunable light source 102, a beam splitter 104a for on-axis beam 118 a, an off-axis ring illuminator 108 for off-axisbeams 118 b, digital imagers 114 and 116, and a processor system 128.

The system 100 a may also be configured, using light beams, to determinea distance between the raised surface feature 113 a and the floorsurface 113 b, with respect to a reference plate 110, of sample 112 thatis part of a semiconductor device 130. According to embodiments, thereference plate 110 may be coated for partial reflectance and partialtransmittance and may be disposed facing and in close proximity (e.g.,100 microns away to a few millimeters away) from the sample 112. The (inone embodiment, static) reference plate 110 may be configured tofunction as a reference mirror of a Mirau interferometer. The referenceplate 110, in this manner, may be positioned above the surface of thesample 112, which may be moved under the optical system 100 a by an x-ytranslation system including a moving stage.

The system 100 a may be deployed in any suitable application where adistance is to be measured. For example, in some embodiments, the raisedsurface feature 113 a may be top surface feature of a semiconductordevice 130 and the floor surface 113 b may be a top or bottom or floorsurface of a silicon substrate wafer that forms a substrate of thesemiconductor device 130. In these and other embodiments, thesemiconductor device 130 may be any circuit, chip, or device that isfabricated on a silicon wafer. The semiconductor device 130 may includemultiple layers of the same or different materials between the raisedsurface feature 113 a and the floor surface 113 b. Alternately oradditionally, the raised surface feature 113 a may be a microelectromechanical systems (MEMS) structure and the floor surface 113 bmay be a surface on which the MEMS structure is built.

Alternately or additionally, the raised surface feature 113 a may be anytype of interconnect feature used in 3D packaging and the floor surface113 b may be the corresponding surface from which the interconnectfeatures protrude. An example of a protruding feature and a referencesurface is described with respect to FIG. 4 . Alternately oradditionally, the raised surface feature 113 a may be an embeddedsurface within a semiconductor device or some other device and thereference surface 113 b may be a top surface. An example of an embeddedsurface is described with respect to FIG. 5 . Although FIGS. 1, 2, 4 and5 illustrate certain feature surface configurations, the principles andoperation of the systems described in FIGS. 1, 2, 4 and 5 may be appliedto any feature surface configuration.

Alternately or additionally, the raised surface feature 113 a may be anytype of interconnect feature used in 3D packaging protruding fromsurface 113 b and the reference surface may be determined relative tothe reference plate 110. Indeed, distances to 113 a and 113 b may bemeasured with respect to the reference plate 110. An example of aprotruding feature and a reference surface is described with respect toFIG. 4 . Alternately or additionally, the raised surface feature 113 amay be an embedded surface within a semiconductor device or some otherdevice and the reference surface may be a top surface. In this manner,the phrase “raised surface feature”, as used herein, may refer tostructures rising above or disposed below the floor or some otherreference surface of the sample. An example of an embedded surface isdescribed with respect to FIG. 4 . Although FIGS. 1, 2, 4 and 5illustrates certain feature surface configurations, the principles andoperation of the systems described in FIGS. 1, 2, 4 and 5 may be appliedto any feature surface configuration.

The tunable light source 102 may be configured to generate and to emit alight beam 118. In some embodiments, the tunable light source 102 may bea broadband light source that is tunable to multiple differentwavelengths. For example, the tunable light source 102 may be tuned in astepwise manner over a range of frequencies. In some embodiments, thetunable light source 102 may have a bandwidth that is between 300nanometers (nm) and 1000 nm, between 1000 nm and 2000 nm, or some otherrange of wavelengths. For example, the tunable light source 102 may havea wavelength bandwidth that is between 650 nm and 950 nm. In someembodiments, the tuning step of the tunable light source 102 may be moreor less than 1 nm. The tunable light source 102 may provide the lightbeam 118 a at a first wavelength to the first beam splitter 104 a.

The first beam splitter 104 a may be optically coupled to the tunablelight source 102, the sample 112, and the digital imager 116. The firstbeam splitter 104 a may be configured to receive the light beam 118 aand to direct or reflect the light beam 118 a towards the sample 112.After being directed by the first beam splitter 104 a, the light beam118 a may strike the reference plate 110 and raised surface feature 113a of the sample 112. Striking the reference plate 110 may generate afirst light beam reflection 121 and striking the raised surface feature113 a of the sample 112 may generate a second light beam reflection 122along the same path as the first light beam reflection 121. Alternatelyor additionally, a portion of the light beam 118 a may traverse throughthe sample 112 to the surface 113 b and strike the surface 113 b.Striking the surface 113 b may generate a third light beam reflection123.

The first light beam reflection 121 and the second beam reflection 122may be directed back toward the first beam splitter 104 a. The thirdlight beam reflection 123 may also be directed back toward the firstbeam splitter 104 a. In these and other embodiments, the first andsecond light beam reflections 121, 122 and the third light beamreflection 123 may coherently add to one another to form a reflectedlight beam 119 a. The tunable light source 102 may also be configured togenerate a second light beam 118 b, which may be directed onto theoff-axis ring illuminator 108. The off-axis ring illuminator 108 may beconfigured to direct the second light beam 118 b toward the referenceplate 110 and the sample 112 at angles other than normal to the surfaceof the reference plate 110 and of the floor of the sample 112. Off-axisincident light may then be reflected/scattered by, for example, raisedsurface features on the sample as reflected beam 119 b, as shown in FIG.1 .

The first beam splitter 104 a may be configured to receive the reflectedlight beam 119 a and the reflected off axis beam 119 b and pass thereflected light beam 118 a and the towards the digital imager 116 overinterferometer channel 122 a. In one embodiment, the reflected lightbeam 118 a and the reflected off-axis beam 118 b may hit the first beamsplitter 104 a, which then transmits a portion thereof though to thesecond beam splitter 104 b. The second beam splitter 104 b may beconfigured to reflects at least a portion of the reflected/scatteredoff-axis light beam 119 b over an imaging channel 122 b to the digitalimager 114 and to transmit at least a portion of the reflected lightbeam 119 b over an interferometer channel 122 a to the digital imager116. In the case in which the tunable light source 102 does not generatethe second light beam 118 b and the off-axis ring illuminator 108 is notpresent, the second beam splitter 104 b may direct a portion of thereflected light beam 119 a toward the digital imager 114 and may directa portion of the reflected light beam 119 a toward the digital imager116. The second beam splitter 104 b may be configured for 50%transmittance (toward digital imager 116) and 50% reflectivity (towarddigital imager 114), or some other transmittance/reflectance ratio.

When the tunable light source 102 does generate the second light beam118 b (of a different wavelength/wavelengths than the first light beam118 a) for darkfield inspection and the off-axis ring illuminator 108 ispresent, the beam splitter 104 b may include a dichroic mirrorconfigured to primarily direct reflections from the first light beam 118(having a first wavelength or band of wavelengths) to the digital imager116 for interferometry and to primarily direct the reflections from theoff-axis illumination light beam 118 b (having a second, differentwavelength or band of wavelengths) to the digital imager 114 fordarkfield inspection. The digital imager 114 may be configured toreceive primarily the off-axis reflected light beam 119 b and togenerate an image 124 based on an intensity of the reflected and/orscattered reflected off-axis light beam 119 b. Similarly, the digitalimager 116 may be configured to primarily receive the reflected lightbeam 119 a and to generate an image 126 based on an intensity of thereflected light beam 119 a. In some embodiments, the digital imagers 114and 116 may be CMOS or CCD type imagers or other types of 1D- or2D-array detectors. In these and other embodiments, the first and seconddigital imagers 114, 116 may include multiple pixel elements. The pixelelements may be configured such that, when illuminated, each providesinformation about the intensity of the illumination that is striking thepixel element. The digital imagers 114, 116 may compile the informationfrom the pixels to form the images 124 and 126. The images 124 and 126may thus include the intensity information for each of the pixels. Theseimages 124, 126, when they include intensity information for each pixel,may be referred to as a grayscale digital images, with the constituentgrey levels thereof corresponding to the different levels of intensityof the illumination. As shown, the digital imagers 114 and 116 mayprovide the images 124 and 126 to the processor system 128.

The processor system 128 may be electrically coupled to the digitalimagers 114 and 116. In these and other embodiments, the processorsystem 128 may receive the images 124 and 126. Based on these images124, 126, the processor system 128 may be configured to determine adistance between the raised surface feature 113 a and the surface orfloor 113 b of the sample 112 and to produce a grayscale inspectionimage of the raised features on the surface of the sample 112. Forexample, the height of the raised surface features above the floorsurface of the sample may be characterized by determining the distancebetween the reference plate 110 and the raised surface feature and thedistance between the reference plate 110 and the floor surface of thesemiconductor. Subtracting the former from the latter may yield theheight of the raised surface feature(s) above the floor surface of thesample.

In some embodiments, the tunable light source 102 may be configured togenerate the light beam 118 a as a point light source with a smalldiameter beam. In these and other embodiments, the area of the sample112 may be small and restricted to a particular location on thesemiconductor device 130. In these and other embodiments, the distancebetween the raised surface feature 113 a and the floor surface 113 b maybe determined for the particular location. Alternately or additionally,the tunable light source 102 may be configured to generate a wider lightbeam 118 a to illuminate a larger field of view. In these and otherembodiments, the area of the sample 112 being imaged may be larger.Indeed, the sample 112 of the substrate of the semiconductor device 130that is illuminated may be several mm² or larger. In these and otherembodiments, the images 124 and 126 may be formed based on the lightbeams 119 a, 119 b reflected or scattered by the topographical featuresor thickness of the sample 112. Thus, the images 124 and 126 may be animage of a portion or may cover an entire area of the sample 112, asopposed to a limited portion of the semiconductor device 130.

In another embodiment, the digital imagers 114 and 116 may be line scancameras scanning across a larger illuminated area on raised surfacefeature 113 a. By moving the semiconductor device 130 continuouslyacross the field of view of the line scan camera, images 124 and 126 ofan entire area of the surface may be gradually built and acquired.Alternatively still, the scan lines may be electrically steered to scana surface or different adjacent scan lines may be activated in turn toscan the intended surface. Other embodiments may occur to those of skillin this art.

In these and other embodiments, particular pixels in the images 124 and126 may correspond to particular locations in the area of the sample112. The processor system 128 may be configured to determine a distancebetween the reference plate 110 and raised surface feature 113 a and adistance between the reference plate 110 and the surface 113 b of thesample 112 at multiple different locations within the area of the sample112. In these and other embodiments, the processor system 128 may useillumination intensity information from particular pixels in the image126 from the digital imager 116 on the interferometer channel 122 a todetermine the distance between the raised surface feature 113 a and thefloor surface 113 b at particular locations of the sample 112 thatcorrespond to the particular pixels in the image 126.

For example, a first pixel or a first group of pixels in the image 126may correspond to a portion of the reflected light beam 119 a thatreflected from reference plate 110 and from a first location of thesample 112. A second pixel or a second group of pixels in the image 126may correspond to a portion of the reflected light beam 119 a thatreflected from reference plate 110 and from a second location of thesample 112. Thus, the first pixel in the image 126 may have a grayscalevalue that is based on the first pixel or on the first group of pixelsin the image 126, based on the intensity of the reflected light beam 119a that reflected from the first location of the sample 112. Furthermore,the second pixel in the image 126 may have a grayscale value that isbased on the second pixel or on the second group of pixels in the image126, based on the intensity of the reflected light beam 119 a thatreflected from the second location of the sample 112.

In these and other embodiments, the processor system 128 may beconfigured to determine the distance between the reference plate 110,the raised surface feature 113 a and the floor surface 113 b at thefirst location of the sample 112 based on the grayscale value(s) of thefirst pixel or the first group of pixels. The processor system 128 mayalso be configured to determine the distance between the reference plate110, the raised surface feature 113 a and the surface 113 b at thesecond location of the sample 112 based on the grayscale value(s) of thesecond pixel or the second group of pixels. In these and otherembodiments, the distance between the reference plate 110 and the raisedsurface feature 113 a and the surface 113 b at the first location andthe second location may be different. In these and other embodiments,based on the different distances between the raised surface feature 113a and the floor surface 113 b from the reference plate 110 at differentlocations of the sample 112, the processor system 128 may generate atopology/topography representation or a data set that is representativeof the topology/topography of the area of the sample 112 that reflectsthe different distances between the raised surface feature 113 a and thefloor surface 113 b at different locations of the sample 112.

The different intensities of the reflected light beam 119 a received bydifferent pixels of the digital imager 116 may result from differentdistances between the raised surface feature 113 a and the floor surface113 b at different locations of the sample 112. The different distancesbetween the raised surface feature 113 a and the floor surface 113 b atdifferent locations of the sample 112 may result in different pathlength differences traversed by the first light beam reflection 121, thesecond light beam reflection 122 and the third light beam reflection 123at different locations of the sample 112. The different path lengthdifferences may result in different phase differences between thereflections from at different locations. The different phase differencesmay result in a change in intensity when the second light beamreflection 122 and the third light beam reflection 123 add coherentlywith first beam reflection 121 from reference plate 110 that iscoaxially-disposed with beam 121, to form the reflected light beam 119a. These beams may add coherently and generate an intensity (grayscale)pattern that is dependent on the phase difference between the firstlight beam reflection 121 and second beam reflection 122 and the phasedifference between the first light beam reflection 121 and the thirdlight beam reflection 123. For example, when the reflected light beamsare in-phase, they interfere constructively (strengthening inintensity). Conversely, when the reflected beams are out-of-phase, theyinterfere destructively (weakening in intensity). These changes inintensity differences may be represented by the different grayscalevalues of the pixels in the image 126.

An example of the operation of the system 100 a is now described. Thetunable light source 102 may be configured to generate and to emit anumber of different light beams 118 a, 118 b. Each of the multipledifferent light beams 118 a, 118 b may be generated at a different timeand at a different wavelengths within predetermined wavelength ranges.In some embodiments, the different wavelengths of the different lightbeam 118 a, 118 b may result in different intensities of the reflectedlight beams 119 a, 119 b. The different intensities may be due to thedifferent wavelengths of the different light beams 118 a, 118 b causingdifferences in the phase between the second light beam reflection 122and the third light beam reflection 123 when added coherently to thefirst beam reflection 121 from the reference plate 110. For example, ata first wavelength of the light beam 118 a, the coherently added firstlight beam reflection 121 from reference plate 110 and the second lightbeam reflection 122 and the coherently added first light beam reflection121 from reference plate 110 and the third light beam reflection 123 mayhave a first phase difference at a second wavelength of the light beam118 a, the coherently added first light beam reflection 121 fromreference plate 110 and the second light beam reflection 122 and thecoherently added first light beam reflection 121 from reference plate110 and the third light beam reflection 123 may have a second phasedifference. The coherent addition with different phase differences maycause the reflected light beam 1198 a to have different intensities.According to one embodiment, the number of the light beams emitted bythe tunable light source 102 may be selected based on the distancebetween the surface of the reference plate 110 and the raised surfacefeatures of the sample 112. The determined distances between thereference plate 110 that faces the sample 112 and the raised surfacefeatures of the sample 112 may be based on a plurality of imagesgenerated based on the plurality of light beams.

Each of the different reflected light beams 119 a may be used by thedigital imager 116 to generate a different image 126. The processorsystem 128 may receive and store each of the different images generatedby the digital imager 116. The processor system 128 may then use thedifferent received images 126 to determine the distance between theraised surface feature 113 a and the floor surface 113 b.

In some embodiments, the processor system 128 may use the differentintensities of the reflected beams 119 a of different images todetermine the distance between the raised surface feature 113 a and thefloor surface 113 b. For example, in some embodiments, the processorsystem 128 may extract the grayscale value, representing an intensityvalue, for a corresponding (e.g., same) pixel of each image 126. Thecorresponding pixel in each image 126 may correspond with a particularpixel element in the digital imager 116. Thus, a particular pixel ineach image 126 may be generated from the same pixel element in thedigital imager 116. The grayscale values for the particular pixel ineach image 126 may be plotted to form a fringe pattern with a sinusoidalwaveform, a modulated sinusoidal waveform, or a complex waveform. Forexample, the intensity values of a particular pixel from differentimages may be plotted along the y-axis and the wavelength of the lightbeam 118 a used to generate the different images may be plotted alongthe x-axis. In these and other embodiments, the distance between thereference plate 110 and the raised surface feature 113 a and thedistance between the reference plate and floor surface 113 b at aparticular point corresponding to the particular pixel may be determinedbased on the plotted fringe patterns.

For example, in some embodiments, the distance between the referenceplate 110 and the raised surface feature 113 a and the distance betweenthe reference plate 110 and floor surface 113 b at a particular pointcorresponding to a particular pixel may be determined based on afrequency domain transformation, such as a Fast Fourier Transform (FFT),a Discrete Fourier Transform (DFT), a Discrete Cosine Transform (DCT) orHilbert Transform of the fringe patterns. Other transformations may beutilized. Alternately or additionally, in some embodiments, thesedistances at a particular point corresponding to a particular pixel maybe determined based on a comparison between a model-based predictedfringe pattern and the determined pixel intensity fringe patterns fromthe images 126. Each of the model-based predicted fringe patterns may beconstructed for a different distance, based on previous actual resultsor theoretical mathematical expressions. For example, a relationshipbetween a phase difference and an intensity of reflected light beam 119a may be determined or estimated using the following mathematicalexpression:

$I_{0} = {I_{1} + I_{2} + {2\sqrt{I_{1}I_{2}}{\cos\left( \frac{2\pi d}{\lambda} \right)}}}$

In the above expression, “I₁” refers to the intensity of the first lightbeam reflection 121 from reference plate 110, “I₂” may refer to theintensity of the second light beam reflection 122 from the raisedsurface feature 113 a, “d” may refer to the optical distance between thereference plate 110 and the raised surface feature 113 a, “A” refers tothe wavelength of the light beam 118 a, and “I₀” may refer to themeasured intensity of the reflected light beam 119 a. In the aboveexpression, “I₂” may also refer to the intensity of the third light beamreflection 123 from the floor surface 113 b, “d” may refer to theoptical distance between the reference plate 110 and the floor surface113 b, “λ” may refer to the wavelength of the light beam 118 a, and “I₀”may refer to the intensity of the reflected light beam 119 a bycoherently adding the first light beam reflection 121 and the thirdlight beam reflection 123. Based on the above expression, model basedpredicted fringe patterns may be created for determining the opticalheight of the feature on sample 130.

In these and other embodiments, the fringe pattern determined fromprocessor system 128 may be compared to each or some of the model-basedpredicted fringe patterns. The model-based predicted fringe patternsclosest to the determined fringe pattern may be selected and thedistance for which the selected model based predicted fringe wasconstructed may be the determined distance between the raised surfacefeature 113 a and the floor surface 113 b.

In some embodiments, the processor system 128 may perform an analogousanalysis for each pixel of the different images 126. Using the distanceinformation from each pixel, the processor system 128 may determine thetopology of the area of the sample 112 illuminated by the light beam 118a.

In some embodiments, a number of different light beams 118 a withdifferent wavelengths used by the system 100 a and thus a number ofdifferent images generated by the digital imager 116 may be selectedbased on an estimated distance between the raised surface feature 113 aand the floor surface 113 b. When the optical distance between theraised surface feature 113 a and the floor surface 113 b is small, suchas below 1 micrometer (μm), the number of different light beams 118 amay be increased as compared to when the optical distance between theraised surface feature 113 a and the floor surface 113 b is larger, suchas above 1 μm. In these and other embodiments, an inverse relationshipbetween the distance to be determined between the two surfaces and thenumber of different light beams 118 a may exist. As such, a bandwidth ofthe wavelengths of the different light beams 118 a may have an inverserelationship with the distance to be determined between the raisedsurface feature 113 a and the floor or reference surface 113 b, asshorter distances may call for an increased number of differentwavelengths to correctly characterize the small height of the raisedsurface features of the sample 112, and vice-versa.

The relationship between the distance to be determined between theraised surface feature 113 a and the floor surface 113 b and thedifference in the wavelength between respective different light beams118 a (the wavelength step size) may also have an inverse relationship.Thus, for a small size distance between the raised surface feature 113 aand the floor surface 113 b, the wavelength step-size may be a firstwavelength step-size. For a medium size distance between the raisedsurface feature 113 a and the floor surface 113 b, the wavelengthstep-size may be a second wavelength step-size and for a large sizedistance between the raised surface feature 113 a and the floor surface113 b, the wavelength step-size may be a third wavelength step-size. Inthese and other embodiments, the third wavelength step-size may besmaller than the first and second wavelength step-size and the secondwavelength step-size may be smaller than the first wavelength step-size.Additionally, the bandwidth of each light beam 118 a corresponding toeach wavelength step may get smaller as the distance between the raisedsurface feature 113 a and the floor surface 113 b increases.

In some embodiments, the semiconductor device 130 may be repositionedwith respect to the system 100 a. For example, the semiconductor device130 may be moved or the system 100 a may be moved. Both may be movedrelative to one another. In these and other embodiments, the system 100a may be configured to determine a distance between the raised surfacefeature 113 a and the floor surface 113 b from a second sample of thesemiconductor device 130. The second sample of the semiconductor device130 may have been a portion of the semiconductor device 130 that waspreviously unilluminated by the light beam(s) 118 a, 118 b or for whichreflections from second sample did not reach the digital imagers 114 and116. In these and other embodiments, the semiconductor device 130 may berepositioned such that entire surface of the semiconductor device 130may be a sample for which the distance between the raised surfacefeature 113 a and the floor surface 113 b is determined or image of thesample surface is captured. In these and other embodiments, the system100 a may be repositioned such that entire surface of the semiconductordevice 130 may be a sample for which the distance between the raisedsurface feature 113 a and the floor surface 113 b is determined or imageof the sample surface is captured.

Modifications, additions, or omissions may be made to the system 100 a.For example, in some embodiments, the system 100 a may includeadditional optical components between the first and second beamsplitters 104 a, 104 b and the digital imagers 114 and 116, asillustrated in FIG. 2 .

The system 100 a as shown and described herein differs from previousdistance measurement concepts. Advantageously, according to embodiments,because the reference plate 110 and both the raised surface feature 113a and the surface 113 b are illuminated by the same light beam 118 a,any vibrations of the semiconductor device 130 affects all beamreflected from sample 130 substantially equally and at substantially thesame time such that the system 100 a may compensate for the vibrations.

In some embodiments, an interferometer system may include multipletunable light sources, multiple beam splitters, and multiple digitalimagers. In some embodiments, an interferometer system may includesingle tunable light sources, multiple beam splitters, and digitalimagers. In these and other embodiments, a tunable light source, a beamsplitter, and a digital imager may be referred to in this disclosure asinterferometer sub-systems.

FIG. 1A illustrates an embodiment of an inspection system 100 a (the“system 100 a”). In general, the system 100 a may be configured to imagea raised surface feature 113 a of a sample 112 that is part of asemiconductor device 130 using light beams 118 a and 118 b. To image thesurface, the system 100 a may include a tunable light source 102, beamsplitters 104 a and 104 b, lens system 106, a digital imager 114, and aprocessor system 128.

The system 100 a may be implemented to generate an image of the raisedsurface features 113 a and floor surface 113 b by illuminating thesurfaces 113 a, 113 b with light beams 118 a and 118 b. The two beamsmay be generated simultaneously or may be generated one after another.

In some embodiments, the imaging system 100 a may include light beam 118b, an off-axis ring illuminator 108, a second beam splitter 104 b and adigital imager 114. This embodiment enables generation of dark fieldimages of the sample surface 113 a. The off-axis ring illuminator 108,as shown in FIG. 1 , transmits the light beam 118 b toward the sample112 at an angle (other than normal) relative to the reference plate 110and the sample 112, as opposed to the normal incidence light beam 118 a,which is directed toward the reference plate 110 and the sample 112perpendicularly, or substantially perpendicularly, as shown in FIGS. 1A,2 and 3 .

In some embodiments, the imaging system 100 a may include light beam 118a, first and second beam splitters 104 a and 104 b, imaging lens system106 and digital imager 114. This embodiment enables generation ofbrightfield field images of the surface of the sample surface.

In other embodiments, the imaging system 100 a may include both lightbeams 118 a and 118 b, both first and second beam splitters 104 a and104 b, and a digital imager 114. This embodiment enables generation ofbrightfield field images of the surface of the sample.

In still further embodiments, the imaging system 100 a may include bothlight beams 118 a and 118 b, both first and second beam splitters 104 aand 104 b, and both digital imagers 114 and 116. This embodiment enablesgeneration of brightfield field images of the surface of the sample.

The system 100 a may be implemented to generate a fluorescence image ofthe surface 113 a. In some embodiment, a fluorescence image of thesurface 113 a is generated by illuminating the surface 113 a with lightbeams 118 a and 118 b composed of one or more shorter wavelengths fromthe tunable source 102. This shorter wavelength may excite materials ofsurface 113 a to fluoresce at correspondingly longer wavelengths. Thefluorescence emission from surface 113 a may be imaged by both digitalimagers 114 and 116.

In some embodiment, a fluorescence image and bright field/dark fieldimage of the surface 113 a may be generated by illuminating the surfaceof the sample with light beams 118 a and 118 b composed of shorterwavelengths from the tunable source 102. This shorter wavelength mayexcite materials of surface to fluoresce at correspondingly longerwavelengths. The fluorescence emission along with reflected andscattered light from the surface of the sample may be imaged by bothdigital imagers 114 and 116, thereby enabling darkfield inspection,brightfield inspection and fluorescence inspection to occursimultaneously in a single device. For example, the tunable light source102 may be configured to generate a first light beam, 118 a having awavelength of, in this example, 520 nm. Interaction of this wavelengthof light with the sample 112 and the raised features thereof causesemission of fluorescence light at a comparatively longer wavelength,such as 620 nm in this example. The 620 nm light, as well as thatportion of the 520 nm light that did not cause fluorescence, aredirected back through the lens system 106, through the first beamsplitter 104 a toward the second beam splitter 104 b. In thisembodiment, the second beam splitter 104 b may be a dichroic mirrorconfigured to primarily direct the fluorescence signal at 520 nm to theimaging channel 122 b to imager 114 for fluorescence imaging andinspection and to primarily direct the reflected 520 nm (with phasechange) light beam to digital imager 116 for brightfield inspection, orvice versa. A third digital imager and a third dichroic mirror may beadded to primarily direct reflections from the second light beam 118 bto third digital imager for darkfield inspection of the surface featuresof the sample 112, to thereby enable brightfield, darkfield andfluorescence inspection simultaneously, in one device.

The system 100 a may be implemented with respect to any suitableapplication where a surface inspection may be required. For example, insome embodiments, the raised surface feature 113 a may be or comprise abump feature of a semiconductor advanced packaging device 130. In theseand other embodiments, the semiconductor device 130 may be any circuit,chip, device, or 3D feature that is fabricated on a silicon wafer. Thesemiconductor device 130 may include multiple layers of the same ordifferent materials between the raised surface feature 113 a and thesurface 113 b. Alternately or additionally, the raised surface feature113 a may be a MEMS structure and the surface 113 b may be a surface onwhich the MEMS structure is built.

Alternately or additionally, the raised surface feature 113 a may be anytype of interconnect feature used in 3D packaging and the surface 113 bmay be the corresponding surface from which the interconnect featuresprotrude. An example of a protruding feature and a reference surface isdescribed with respect to FIG. 4 . Alternately or additionally, theraised surface feature 113 a may be an embedded surface within asemiconductor device or some other device and the reference surface maybe a top surface as shown in FIG. 5 . Although FIGS. 1, 2, 4 and 5illustrate certain surface feature configurations, the principles andoperation of the systems described in FIGS. 1, 2, 4 and 5 may be appliedto any surface feature configuration.

The digital imagers 114 and 116 may be configured to receive thereflected light beam 119 a, 119 b and to correspondingly generaterespective images 124, 126 based on an intensity of the respectivereflected light beams 119 a, 119 b. In some embodiments, the digitalimagers 114, 116 may be CMOS or CCD type imager or other types of 2D-and 1D-array detectors. In these and other embodiments, these digitalimagers may include multiple pixel elements. The pixel elements may beconfigured such that, when illuminated, each pixel element providesinformation about the intensity of the illumination that is striking thepixel element. The digital imagers 114, 116 may compile the informationfrom the pixel elements to form the images 124, 126, respectively. Theseimages may thus include the intensity information for each of the pixelelements. The images 124, 126, when including the intensity informationfor each pixel element, may be referred to as a grayscale digitalimages. The digital imager 114 may provide the image 124 to theprocessor system 128 and digital imager 116 may provide the image 126 tothe processor system 128.

FIG. 1B illustrates multiple interferometer sub-systems 160 a and 160 bin an exemplary interferometer system 100 b, arranged according to atleast some embodiments described in this disclosure. Each of thesub-systems 160 a and 160 b may include a tunable light source, one ormore beam splitters, lens system and digital imagers analogous to thetunable light source 102, the first and second beam splitters 104, lenssystem 106 and the digital imagers 114, 116 of FIG. 1A. Each of thesub-systems 160 a and 160 b may be configured to illuminate a differentportion of a semiconductor device or other sample 180. Images generatedby each of the sub-systems 160 a and 160 b may be provided to aprocessor system 190 that may be analogous to the processor system 128of FIG. 1A. The processor system 190 may be configured to determineheight of the surface features of the semiconductor device 180 based onthe images from the sub-systems 160 a and 160 b. The processor system190 may be configured to generate an image of the surface of sample 180.Thus, in these and other embodiments, multiple samples of thesemiconductor device 130 may be processed at the same time, in parallel.By processing multiple samples at the same time, a height of the raisedsurface features across the semiconductor device 180 may be determinedin less time than when portions of the semiconductor device 180 areprocessed sequentially or one at a time. Similarly, by processingmultiple samples simultaneously, images across the semiconductor device160 may be generated in less time than when successive portions of thesemiconductor device 180 are processed sequentially, one at a time.

Modifications, additions, or omissions may be made to the system 100 bwithout departing from the scope of the present disclosure. For example,each of the sub-systems 160 a and 160 b may include a processor system.In these and other embodiments, one of the processor systems may compileinformation for the entire semiconductor device 180 from other of theprocessor systems.

FIG. 2 illustrates another example interferometer system 200 a (the“system 200 a”), according to at least some embodiments described inthis disclosure. The system 200 a may be configured, using light beams218 a, 218 b, to determine a height of a raised surface feature 213 afrom floor surface 213 b on a sample 212 that is part of a semiconductordevice 230. To determine the distance or capture an image, the system200 a may include a tunable light source 202, first and second beamsplitters 204 a, 204 b, a first lens system 206, digital imagers 224 and226, and a processor system 228.

The system 200 a may be implemented with respect to any suitableapplication where a distance may be measured. For example, in someembodiments, the raised surface feature may be a top surface of asemiconductor device 230 and the floor surface 213 b may be a topsurface of a silicon substrate wafer that forms a substrate of thesemiconductor device 230 from which the raised surface features rise.

The tunable light source 202 may be configured to generate and to emit alight beam 218 a and light beam 218 b. The tunable light source 202 maybe analogous to the tunable light source 102 of FIG. 1A and may beconfigured to provide light beams 218 a, 218 b at a particularwavelength or at several different wavelengths within a range ofwavelengths over a period of time. As illustrated in FIG. 2 , in someembodiments, the tunable light source 202 may include a broadband lightsource 222 and a tunable filter 210 that are co-axially disposed andoptically coupled. The broadband light source 222 may be configured toemit a broadband light beam 216 that includes wavelengths of light thatmay be used by the system 200 a. In some embodiments, the broadbandlight source 222 may be a light source such as a white light or a superluminescent diode (SLED). In some embodiments, the broadband lightsource 222 may be configured to provide the broadband light beam 216with a Gaussian power spectrum.

The tunable filter 210 may be configured to filter the broadband lightbeam 216 to generate the light beam 218 a at a particular wavelength. Insome embodiments, the tunable filter 210 may be tuned, such that thetunable filter 210 may filter different wavelengths of light to generatethe light beam 218 a at multiple different wavelengths of light.

An off-axis ring illuminator 208 may be provided and receive light beam218 b also generated by broadband light source 222 to illuminate thesample 212 with off-axis (e.g., not normal incidence) light beam 218 b.In some embodiments, the first beam splitter 204 a may be configured toreceive the light beam 218 a and to direct the light beam 218 a, towardsthe sample 212 via lens system 206. The first beam splitter 204 a may befurther configured to reflect and transmit a portion of the light beam218 a, 218 b. For example, the first and second beam splitters 204 a,204 b may reflect 50 percent and transmit 50 percent of the light beams218 a, 218 b. Alternately or additionally, the beam splitters 204 mayreflect a different percent of the light beams 218 a, 218 b (such as toreflect all or substantially all of the first light beam 218 a). Inthese and other embodiments, the reflected portion of the light beam 218a reflected by first beam splitter 204 a may be directed to the sample212.

The sample 212 may be analogous to the sample 112 in FIG. 1A. In theseand other embodiments, the light beams 218 a, 218 b may be reflectedand/or scattered by the raised surface feature 213 a and the floorsurface 213 b of the sample 212 to form the reflected light beams 218 a,218 b. The reflected light beams 218 a, 218 b may be received by thefirst and second beam splitters 204 a, 204 b via first lens system 206.The first and second beam splitters 204 a, 204 b may reflect a portionand transmit a portion of the reflected light beams 218 a, 218 b.

The first lens system 206 may be configured to receive the reflectedlight beams 218 a and 218 b from sample 212 and to focus images of thesample on the digital imagers 224, 226. A second lens system 207 may beprovided between the first and second beam splitters 204 a, 204 b. Thefirst lens system 206 and the second lens system 207, if present, maypass and focus the reflected light beam 218 a, 218 b toward the digitalimagers 224 and 226. The digital imagers 224, 226 may include an imagesensor. The image sensor may include a CMOS image sensor, a CCD imagesensor, or other types of 1D- and 2D-array detectors. The digitalimagers 224 and 226 may generate images 234 and 236 based on thereflected and scattered light beams 218 a, 218 b and pass the images tothe processor system 228.

The processor system 228 may be analogous to and configured to operatein a similar manner as does the processor system 128 of FIG. 1A. Theprocessor system 228 may be implemented by any suitable mechanism, suchas a program, software, function, library, software as a service,analog, or digital circuitry, or any combination thereof. In someembodiments, such as illustrated in FIG. 2 , the processor system 228may include a data acquisition module 250 and a processor and storagemodule 252. The processor and storage module 252 may include, forexample, a microprocessor, microcontroller, digital signal processor(DSP), application-specific integrated circuit (ASIC), aField-Programmable Gate Array (FPGA), or any other digital or analogcircuitry configured to interpret and/or to execute program instructionsand/or to process data. In some embodiments, the processor of theprocessor and storage module 252 may interpret and/or execute programinstructions and/or process data stored in the memory of the processorand storage module 252. For example, the images 234 and 236 generated bythe digital imagers 224 and 226 may be stored in the memory of theprocessor and storage module 252. The processor of the processor andstorage module 252 may execute instructions to perform the operationswith respect to the image 234 and 236 to generate image of sample 212and to determine the distance between the raised surface feature 213 aand the floor surface 213 b and/or thickness of the sample 212

The memory in the processor and storage module 252 may include anysuitable computer-readable media configured to retain programinstructions and/or data, such as the images 234 and 236, for a periodof time. By way of example, and not limitation, such computer-readablemedia may include tangible and/or non-transitory computer-readablestorage media including Random Access Memory (RAM), Read-Only Memory(ROM), Electrically Erasable Programmable Read-Only Memory (EEPROM),Compact Disc Read-Only Memory (CD-ROM) or other optical disk storage,magnetic disk storage or other magnetic storage devices, flash memorydevices (e.g., solid state memory devices), or any other storage mediumwhich may be used to carry or store desired program code in the form ofcomputer-executable instructions or data structures and which may beaccessed by a general-purpose or special-purpose computer. Combinationsof the above may also be included within the scope of computer-readablemedia. Computer-executable instructions may include, for example,instructions and data that cause a general-purpose computer,special-purpose computer, or special-purpose processing device toperform a certain function or group of functions. Modifications,additions, or omissions may be made to the system 200A without departingfrom the scope of the present disclosure.

The embodiment of the interferometer system 200 a of FIG. 2 may includean adjustable aperture device 232, which may be adjusted, according toone embodiment, based upon the lateral resolution of the interferometersystem 200 a.

The adjustable aperture device 232 may be configured to adjust a size ofan aperture 242 through which the reflected light beams 219 a, 219 b maytravel. In some embodiments, the adjustable aperture device 232 may bepositioned between the first and second beam splitters 204 a and 204 b.The adjustable aperture device 232 may also be positioned between thefirst lens system 206 and the first beam splitter 204 a. Alternately oradditionally, as shown in FIG. 2 , the adjustable aperture device 232may be positioned between the first lens system 206 and tube lenses ofthe digital imagers 224 and 226. In some embodiments, the aperture 242of the adjustable aperture device 232 may result in an adjustable systempupil plane. A position of the adjustable system pupil plane may bebased on a position of the adjustable aperture device 232 (and thereforethe size of aperture 242) in the imaging path made up of lens system206, aperture 242, and the digital imagers 224, 226. In someembodiments, a position of the adjustable system pupil plane, and thusthe position of the adjustable aperture device 232, may be determinedbased on whether the adjustable system pupil plane is configured tocontrol spatial resolution or field of view of the digital imagers 224,226.

In some embodiments, the size of the aperture 242 may be adjusted basedon a feature size in an area of the sample 212. In some embodiments, thesize of the aperture 242 may be adjusted based on a required spatialresolution of the area of the sample 212 that is being imaged by thedigital imagers 224, 226. In these and other embodiments, adjusting thesize of the aperture 242 may affect one or more of: a light beam angleor a numerical aperture (NA) of the reflected light beam 219 a or 219 bon the first lens system 206; relay of the reflected light beam 219 a,219 b; sharpness of the images 234, 236 generated by the digital imagers224, 226; depth of focus, the field of view and spatial resolution onthe digital images, among others.

In some embodiments, the system 200 a may be configured beforedetermining a distance between the raised surface feature 213 a and thesurface 213 b. In these and other embodiments, the size of the aperture242 may be selected. The size of the aperture 242 may be selected basedon an area of the sample 212. The area of the sample 212 may be an areain a plane that includes at least a portion of a raised surface feature213 a. In these and other embodiments, the size of the aperture 242 maybe based on required lateral resolution, i.e., the smallest size of afeature to be measured within or on the sample 212 of the semiconductordevice 230. In these and other embodiments, when the lateral size of thefeature is small, the size of the aperture 242 is correspondinglylarger. When the lateral size of the feature is larger, the size of theaperture 242 is correspondingly smaller.

In some embodiments, configuring the system 200 a may include setting anexposure time and gain of the digital imagers 224 and 226. In theseother embodiments, an initial exposure time and gain may be selected forthe digital images 234, 236. The initial exposure time and gain may beselected based on the area and the reflectivity of the sample 212. Inone embodiment, the exposure time, and the signal to noise of thedigital imagers 224, 226 may be dependent on digital imagers' read-outtime, the number of wavelengths in the plurality of light beams and thereflectivity of the sample 212.

After selecting the initial exposure time and gain, the light beams 218a, 218 b may illuminate the sample 212 and images 234, 236 may becaptured by the digital imagers 224 and 226 from the reflected lightbeams 219 a, 291 b. The images 234, 236 may be processed to determine ifany pixels thereof saturated after having been exposed to the reflectedlight beam 219 a, 219 b. Saturation may be determined when there is flatline of a grayscale value across multiple adjacent pixels in the images234, 236. When it is determined that some of the pixels of the images234, 236 saturated, the gain and/or the exposure time may be reduced.For example, the gain may be reduced by ten percent. The process ofchecking for saturation by the processor system 228 may be iterativelycarried out and the gain and the exposure time successively reduceduntil the little or no saturation of pixels occurs at a particularwavelength of the light beams 218 a, 218 b. In these and otherembodiments, the particular wavelength selected may be the wavelengthwith the highest power. Using the wavelength with the highest powerduring configuration may reduce the likelihood of saturation of pixelswith wavelengths of lower power during operation of the system 200 a.

In some embodiments, configuring the system 200 a may include selectinga range of wavelengths for the light beams 218 a, 218 b and thewavelength step size between light beams 218 a, 218 b. In someembodiments, the range of wavelengths for the light beams 218 a, 218 band the wavelength step size may be selected based on a shortestdistance between the raised surface feature 213 a and the floor surface213 b over the area of the sample 212. In these and other embodiments,an approximate or estimated shortest distance may be selected based onthe known design and construction of the semiconductor device 230.Indeed, the intended, designed-for smallest and largest features of thesemiconductor device 230 may be known a priori. In these and otherembodiments, the range of wavelengths for the light beams 218 a, 218 band the wavelength step size may then be selected based on the shortestanticipated feature size. As discussed previously, the range ofwavelengths for the light beams 218 a, 218 b and the wavelength stepsize may have an inverse relationship with respect to distance betweenthe raised surface feature 213 a and the floor surface 213 b.

Modifications, additions, or omissions may be made to the system 200 awithout departing from the scope of the present disclosure. For example,in some embodiments, the adjustable aperture device 232 may be locatedbetween the first lens system 206 and the first beam splitter 204 a orbetween the first and second beam splitters 204 a, 204 b, as shown inFIG. 2 .

FIG. 4 illustrates an example of beam reflection off another exemplarysemiconductor device 400, arranged in accordance with at least someembodiments described in this disclosure. As shown therein, referenceplate 410 is positioned facing a distance away from and above thesemiconductor device 400. According to embodiments, the reference plate410 may be coated for partial reflectance and partial transmissivity.The semiconductor device 400 may include a first raised portion 406 aand a second raised portion 406 b, both of which extend above a samplefloor surface 404 of the semiconductor device 400. A top surface of thefirst raised portion 406 a may be a first raised surface feature 402 aof the semiconductor device 400. A top surface of the second raisedportion 406 b may be a second raised surface feature 402 b of thesemiconductor device 400. Using light beams and an interferometer systemdescribed in some embodiments in the present disclosure, a distance D1between the reference plate 410 and the first feature surface 402 a maybe determined. Alternately or additionally, a distance D2 between thereference plate 410 and the surface 404 may be determined. From thesetwo distance measurements, the height of the raised surface feature 406a away from surface 404 may be determined. In some embodiments, thefloor surface 404 may be at varying heights with respect to the firstand second raised portions 406 a and 406 b. For example, the floorsurface 404 to the left of the first raised portion 406 a may be higherthan the floor surface 404 to the right of the second raised portion 406b. Therefore, the height of any raised surface feature of thesemiconductor device 400 may be characterized by its height above astated reference surface. Indeed, that reference surface may be theunderside of the reference plate that faces the semiconductor device 400as shown with reference to D1. Alternatively, that reference surface maybe a predetermined portion of the sample floor surface 404, as shownwith reference to D2.

FIG. 4 illustrates a light beam from a light source TS1. The light beamfrom TS1 includes a first light beam portion 414 a that is incident uponthe reference plate surface 410, thereby generating first reflected beam415 a and a second light beam portion 414 a′ that strikes the surface404. A part of the second light beam portion 414 a′ may be reflected offof the surface 404 and generate a second reflected beam 416 a. The restof the first light beam portion 414 a′ may pass through thesemiconductor device 400 and/or incur additional reflections, scatteringor refractions.

A part of the second light beam portion 414 b from a light source TS2may be reflected off of the reference plate to generate reflected beam415 b and a part of the second light beam portion 414 b may strike theraised surface feature 402 a and generate a second reflected beam 416 b.A remaining portion of the second light beam portion 414 b′ may passthrough the semiconductor device 400 and/or incur additionalreflections, scattering or refractions.

The first and second reflected beams 415 a, 416 a and 415 b, 416 b maycoherently add to form a combined reflected beam 420. In someembodiments, the reflected beam 420 may pass through the first lenssystem 206, the aperture 242 as illustrated and described with respectto FIG. 2 and be provided to the digital imagers 224, 226. Images (suchas shown at 234, 236 in FIG. 2 , for example) may be formed using atleast the intensity of the reflected beam 420. The images thus generatedmay be part of a collection of images that may be stored and used todetermine the distances D1 and D2.

In some embodiments, the light source TS1 may also illuminate the secondraised portion 406 b. In a similar manner as described with respect tothe first raised portion 406 a, a reflected beam may be formed andcaptured to form an image or images. The image may be part of acollection of images that may be used to determine the distances D1 andD2.

FIG. 5 illustrates an illustrative example of beam reflection offanother exemplary semiconductor device 500, arranged in accordance withat least some embodiments described in this disclosure. Thesemiconductor device 500 may include a first raised portion 506 a and asecond raised portion 506 b that extend away from a first surface 508 ofthe semiconductor device 500 toward a reference plate 504. A top surfaceof the first raised portion 506 a may be a first raised surface feature502 a of the semiconductor device 500. A top surface of the secondraised portion 506 b may be a second raised surface feature 502 b of thesemiconductor device 500. Using light beams and an interferometer systemdescribed in some embodiments in the present disclosure, a distance D1between the reference plate 504 and the first feature surface 502 a maybe determined. The distance D1 may represent a distance that the firstfeature surface 502 a is below (at least in the orientation shown inFIG. 5 ) the reference plate 504. Alternately or additionally, adistance D2 between the reference plate 504 and the second raisedsurface feature 502 b may be determined. In some embodiments, thereference plate 504 may be at varying heights with respect to the firstand second raised portions 506 a and 506 b or with respect to thereference plate 504. In some embodiments the reference plate may not bepart of the semiconductor device 500 or part of substrate 508.

FIG. 5 illustrates a light beam 514 a generated from a light source TS1.The light beam 514 a may strike the reference plate 504. A part of thelight beam 514 a may be reflected off the reference plate 504 andgenerate a first reflected beam 516 a. The rest (or some remainingportion) of the light beam 514 a may pass through the reference plate504 and generate a refracted beam 514 b. The refracted beam 514 b mayhit the first raised surface feature 502 a of the first raised portion506 a and part of the refracted beam 514 b may be reflected off thefirst features surface 502 a to generate a second reflected beam 516 b.The second reflected beam 516 b may pass through the reference plate504, although some additional reflections, refractions and scatteringmay also occur.

The first and second reflected beams 516 a and 516 b may coherently addto form a reflected beam 520. In some embodiments, the reflected beam520 may pass through the first lens system 206, and the aperture 242, asillustrated and described with respect to FIG. 2 and be provided to thedigital imagers 224, 226. Images (such as shown at 234, 236, forexample) may be formed using at least the intensity of the reflectedbeam 520. The thus formed images may be part of a collection of imagesthat may be used to determine the distance D1.

In some embodiments, the light source TS1 may also illuminate the secondraised portion 506 b. In a similar manner as described with respect tothe first raised portion 506 a, a reflected beam may be formed andcaptured to form an image or images. These images may be part of acollection of images that may be used to determine the distance D2.

One skilled in the art will appreciate that, for this and otherprocesses and methods disclosed in this disclosure, the functionsperformed in the processes and methods may be implemented in differingorder. Furthermore, the outlined steps and operations are only providedas examples, and some of the steps and operations may be optional,combined into fewer steps and operations, or expanded into additionalsteps and operations within the scope of the disclosed embodiments.

For example, in some embodiments, the implementation method may includeadjusting a size of an aperture (see reference 242 in FIG. 2 , forexample), through which the reflected light beam passes, based on anarea of a feature along the feature surface for which the distancebetween the reference plate and the feature surface is determined.

In these and other embodiments, a wavelength difference between thefirst wavelength and the second wavelength emitted simultaneously orover time may be selected based on the height or depth of the raisedsurface feature on the sample surface relative to the reference plate110 or the floor of the sample 112. The pathlength inside the referenceplate 110 is not significant and both facing surfaces of the referenceplate contribute to that pathlength. In these and other embodiments,determining the height or depth of raised surface features on samplesurface based on wavelength dependent images may include constructing awaveform or fringe pattern based on wavelength dependent intensityvalues and performing a frequency domain transformation such as a FastFourier Transform or Hilbert Transform on the waveform or fringepattern. In these and other embodiments, the distance between thedifferent surfaces at a first location on the sample may be determinedbased on a first intensity value at a first pixel location in the firstimage and a second intensity value at the first pixel location in thesecond image. In these and other embodiments, the distance may be afirst distance and the implementation method may further includedetermining a second distance between the reference plate and the raisedsurface feature based on the first image and the second image at asecond location on the sample. The second distance may be determinedbased on a first intensity value at a second pixel location in the firstimage and a second intensity value at the second pixel location in thesecond image.

In these and other embodiments, the imaging lens system 106 shown inFIG. 1A or the imaging lens system 206 in FIG. 2 is common for both theimaging channel 122 b and the interferometer channel 122 a. Alternatelyor additionally each channel 122 a, 122 b or 222 a, 222 b may have itsown, independent imaging lens system.

FIG. 3 illustrates another embodiment of an interferometer system 300 a(the “system 300 a”). The system 300 a may be configured to determine,using light beams, a height of a raised surface feature 313 a away froma floor or reference surface 313 b of a sample 312 that is part of asemiconductor device 330. To determine the distance or to capture animage, the system 300 a may include a tunable light source 302, firstand second beam splitters 304 a, 304 b, a first lens system 306, anauto-correlation interferometer 340, digital imagers 334 and 336, and aprocessor system 338.

The system 300 a may be implemented with respect to any suitableapplication where a distance may be measured. For example, in someembodiments, the raised surface feature 313 a may be a top surface of asemiconductor device 330 and the floor surface 313 b may be a topsurface of a silicon substrate wafer that forms a substrate of thesemiconductor device 330.

The tunable light source 302 may be configured to generate and to emitlight beams 318 a, 318 b. The tunable light source 302 may be analogousto the tunable light source 102 of FIG. 1A and may be configured toprovide a light beam 318 a at a particular wavelength. An off-axis ringilluminator 308 may be provided and receive light beam 318 b toilluminate the sample 312 with off-axis light beam 318 b. As illustratedin FIG. 2 , in system 300 a and in some embodiments, the tunable lightsource 302 may include a broadband light source and a tunable filterthat are co-axially-disposed and optically coupled. The broadband lightsource may be configured to emit a broadband light beam that includeswavelengths of light that may be used by the system 300 a. In someembodiments, the broadband light source may be a light source such as awhite light or a super luminescent diode (SLED). In some embodiments,the broadband light source may be configured to provide the broadbandlight beam with a Gaussian power spectrum.

The tunable light source 302 may be configured to generate the lightbeam 318 a at a particular wavelength. In some embodiments, the tunablelight source 302 may be tuned, to generate different wavelengths oflight to generate the light beam 318 a at multiple different wavelengthsof light within a predetermined range of wavelengths. The tunable lightsource 302 also may be configured to generate the light beam 318 b at aparticular wavelength. In some embodiments, the tunable light source 302may be tuned to generate different wavelengths of light to generate thelight beam 318 b at multiple different wavelengths of light within apredetermined range of wavelengths. These different wavelengths may beprovided to the off-axis ring illuminator 308 over a period of time, toenable, among other applications, dark-field images of the sample 312.

In some embodiments, at least the first and beam splitters 304 a may beconfigured to receive the light beam 318 a and to direct the light beam318 a towards the sample 312 via first lens system 306. The first andsecond beam splitters 304 a, 304 b may be configured to reflect andtransmit a portion of the light beam 318 a, 318 b. For example, thefirst and second beam splitters 304 a, 304 b may reflect 50 percent andtransmit 50 percent of the light beam 318 a. Alternately oradditionally, the first and second beam splitters 304 a, 304 b mayreflect a different percentage of the light beam 318 a, 318 b incidentthereon. In these and other embodiments, the portion of the light beam318 a reflected by the first beam splitter 304 a may be directed to thesample 312.

The sample 312 may be analogous to the sample 112 in FIG. 1A. In theseand other embodiments, the light beams 318 a, 318 b may be reflected bythe reference plate 310, the raised surface feature 313 a and the floorsurface 313 b of the sample 312 to form the reflected light beams 319 a,319 b. According to embodiments, the reference plate 310 may be coatedfor partial reflectance and partial transmittance. The reflected lightbeams 319 a 319 b, reflected and/or scattered by sample 330, may bereceived by the first and second beam splitters 304 a and 304 b viafirst lens system 306. A second lens may be provided between the beamsplitters 304 a, 304 b. The first beam splitter 304 a may be configuredto transmit at least a portion of the reflected light beams 319 a, 319 btowards the second beam splitter 304 b. The second beam splitter 304 b,in turn, may reflect a portion and transmit a portion of the reflectedlight beam 319 a. The reflected beam 319 a reflected by second beamsplitter 304 b, in this embodiment, is directed toward anauto-correlation interferometer 340 over an auto correlation channel 323to implement auto-correlation of reflections from the reference plate310, and the surfaces 313 a and 313 b. The digital imager 334 is alsocoaxially disposed along the auto correlation channel 323 and isconfigured to receive light transmitted by the auto correlationinterferometer 340 through the second beam splitter 304 b. In the system300 a, the auto-correlation interferometer 340 may include a cavityhaving movable mirrors 341, 342. The mirrors 341 or 342 ofauto-correlation interferometer 340 are movable along the optical axissuch that the path differences between reflected light signal fromreference plate 310 and those light reflections from surfaces 313 a and313 b satisfy temporal coherence condition. By translating the mirror341 and/or mirror 342 disposed in the auto-correlation interferometer340, the reference plate reflection from moving the mirrors 341, 342 canbe correlated to the fixed mirror reflections from various surfaces inthe sample 330.

The first lens system 306 may be configured to receive the reflectedlight beam 319 a and the light beam from the reflected and/or scatteredoff-axis light beam 319 b off of the sample 312 through the first beamsplitter 304 a. The reflected light beams 319 a, 319 b are thentransmitted towards the second beam splitter 304 b, which reflects aportion thereof as an auto-correlator beam 319 a on the auto correlationchannel onto the digital imagers 334 and a remaining portion thereofthrough the imaging channel 322 onto the digital imager 336. The digitalimagers 334, 336 may include image sensors. The image sensor may be aCMOS image sensor, a CCD image sensor, or other types of 1D- and2D-array detectors. The digital imagers 334 and 336 may generate images324 and 326 based on the reflected and scattered light beams 319 a, 319b and pass the images to the processor system 338.

The processor system 338 may be analogous to and configured to operatein a similar manner as the processor system 128 of FIG. 1A. Theprocessor system 338 may be implemented by any suitable mechanism, suchas a program, software, function, library, software as a service,analog, or digital circuitry, or any combination thereof. In someembodiments, such as illustrated in FIG. 2 , the processor system 228may include a data acquisition module 250 and a processor and storagemodule 252. The processor and storage module 252 may include, forexample, a microprocessor, microcontroller, digital signal processor(DSP), application-specific integrated circuit (ASIC), aField-Programmable Gate Array (FPGA), or any other digital or analogcircuitry configured to interpret and/or to execute program instructionsand/or to process data.

The systems depicted in FIGS. 1A, 2 and 3 may be used to measure minuteheights of semiconductor features. However, the systems depicted inFIGS. 1A, 2 and 3 may also be configured to measure topologies havingmuch greater height differentials measuring, for example, inmillimeters. FIG. 6 illustrates tunable coherence length stretching thatmay be used in interferometer systems to measure topological featureshaving a greater z-height profiles, according to another embodiment

Referring to FIG. 6 , broadband light source 622 may be configured togenerate and to emit a light beam 616 at a particular wavelength or atseveral different wavelengths within a range of wavelengths over aperiod of time. In some embodiments, the broadband light source 622 anda co-axially disposed and optically coupled tunable filter 610 togetherprovide a tunable light source. The broadband light source 622 may beconfigured to emit a broadband light beam 616 that includes wavelengthsof light that may be used by the system 600 in FIG. 6 . In someembodiments, the broadband light source 622 may be a light source suchas a white light laser or ultra-broadband source such as an arc lamp orlaser driven plasma source or a super luminescent diode (SLED). In someother embodiments, the broadband light source 622 may be configured toprovide the broadband light beam with a Gaussian power spectrum. Thetunable filter 610 may be configured to filter the broadband light beam616 to generate a light beam 618 at a particular wavelength, therebyfunctioning as a X selecting element, as shown in FIG. 6 . In someembodiments, the tunable filter 610 may be tuned, such that the tunablefilter 610 may filter out different wavelengths of light from the lightbeam 616 to generate a light beam 618 having simultaneously orsuccessively multiple different wavelengths of light.

The tunable filter 610, according to an embodiment, may be configured togenerate different wavelengths of light with a finite linewidth at eachwavelength. The linewidth defines the temporal coherence length, L_(c),of beam 618. The coherence length in turn determines the minimum heightthat could be measured by the interferometer channels 122 a, 222 a and322, as described above. Tunable light source 622, 610 may be tuned suchthat light beam at each wavelength step has a definable or predeterminedlinewidth. The coherence length of the beam 618 may be tuned by tuningthe linewidth of the laser beam. In turn, control of the linewidth maybe achieved with interference filters with defined passbands or withgrating elements such as shown at reference 650 in FIG. 6 . In agrating-based wavelength selector, definable linewidth may be defined as

δλ=λ/(m·Σ)

where m is the diffraction order and Σ is the number of groovesintercepted by the incident beam. This can be re-written with knownparameters pitch, p and beam diameter Φ as follows:

δλ=λ·p/(m·Φ)

Coherence length of light in air is

L _(c)=0.66·λ²/δλ

or in terms of pitch and beam diameter,

L _(c)=0.66·m·λ·Φ/p

Thus, for a given pitch and diffraction order, the coherence length oflight beam 619 at any wavelength, λ, may be stretched or tuned,according to an embodiment, by adjusting the diameter of the beamincident on the grating element 650 using a variable beam expander, asshown at 640 in FIG. 6 .

In some embodiment of the invention, the light source 622 may beconfigured to generate beams 619 to accommodate coherence lengthrequirement of surface features with differing z-height profile on thesame sample through control of the wavelengths selected at 610 andthrough control, by the variable beam expander 640, of the diameter ofthe beam incident on the grating 650, thereby enabling imaging ofdiffering z-height ranges on the same or different sample. The beams oflight 619 may be collimated in collimating optics 660 and collinearized(e.g., aligned with one another) at 670 before being incident on sample630. The sample 630 may be moving across the incident beam such that thebeam sweeps across the entire or selected surfaces or areas of thesample or the sample may be immobile, and the optical system shown inFIG. 6 moved across the sample.

FIG. 7 illustrates tunable coherence length stretching with centralwavelength stabilization that may be used in interferometer systems tomeasure topological features having a greater z-height profiles,according to yet another embodiment. As shown in FIG. 7 , an opticalpiezoelectrical device such as, for example, an acousto-optic tunablefilter (AOTF) 710 may be used as the wavelength selector 710. With AOTF,wavelength selection can be achieved in microseconds byvarying/selecting an acoustic wave signal input to the AOTF 710. Asshown, the broadband polychromatic or white light beam 716 emitted bybroadband light source 722 is incident upon the AOTF λ tuner 710, whichenables the rapid selection of a specific wavelength beam 718 from theincident 716 by varying the frequency of the acoustic wave 712 input tothe AOTF 710.

Indeed, in AOTF, the wavelength selection is dependent on the frequencyof the acoustic wave input to and propagating through the crystal in thetunable filter. This frequency is determined by the frequency of thetransducer circuitry. It is possible to achieve drive frequencystability of ˜1 ppm. Hence, a central wavelength, λ_(c), with astability on the order of ˜1 to 10 ppm can be achieved. Furthermore,long term drift in λ_(c), can be avoided by thermoelectrically (TE)cooling the AOTF crystal in the AOTF λ tuner 710, as suggested atreference 711 in FIG. 7 .

Similarly, and as shown and described relative to FIG. 6 , in theembodiment of FIG. 7 , for a given pitch and diffraction order, thecoherence length of light beam 719 at any wavelength, λ, can bestretched or tuned by adjusting the diameter of the beam incident on thegrating element 750 using a variable beam expander, as shown at 740.Indeed, the light source 722 may be configured to generate beams 719 toaccommodate coherence length requirement of surface features withdiffering z-height profile on the same sample through control of thewavelengths selected by AOTF λ tuner 710 and through control, by thevariable beam expander 740, of the diameter of the beam incident on thegrating 750, thereby enabling imaging of differing z-height ranges onthe same or different sample. The beams of light 719 may be collimatedin collimating optics 760 and collinearized (i.e., aligned with oneanother) at 770 before being incident on sample 730. The sample 730 maybe moving across the incident beam such that the beam sweeps the entireor selected surfaces of the sample, or the sample may be immobile andthe optical system shown in FIG. 7 moved across the sample.

In this manner, the embodiment in FIG. 7 that incorporates the AOTF λtuner 710, the variable beam expander 740 and grating element 750enables the generation of a laser beam whose coherence length is tunableand whose central wavelength stable is highly stable. Moreover, theembodiment shown in FIG. 7 enables a broadband source 722 that can beswept to produce a multitude of spectral lines, each with highwavelength stability and coherence length tunability. The height of theraised surface (or receded surface as the two terms are usedinterchangeably herein) of the sample may then be measured in a mannerand using devices and structures that are similar to that discussedrelative to FIGS. 1A and 3 , based upon the intensities of the lightreflected from the sample.

Returning now to FIG. 1 , in some embodiments, the processor system 128may use the different intensities of the reflected beams 119 a ofdifferent images to determine the distance between the raised surfacefeature 113 a and the floor surface 113 b. For example, in someembodiments, the processor system 128 may extract the grayscale value,representing an intensity value, for a corresponding (e.g., same) pixelof each image 126. The corresponding pixel in each image 126 maycorrespond with a particular pixel element in the digital imager 116.Thus, a particular pixel in each image 126 may be generated from thesame pixel element in the digital imager 116. The grayscale values forthe particular pixel in each image 126 may be plotted to form a fringepattern with a sinusoidal waveform, a modulated sinusoidal waveform, ora complex waveform. For example, the intensity values of a particularpixel from different images may be plotted along the y-axis and thewavelength of the light beam 118 a used to generate the different imagesmay be plotted along the x-axis, as shown in FIG. 8A. In these and otherembodiments, the distance D₁ between the reference plate 110 and theraised (or receded) surface feature 113 a and the distance D₂ betweenthe reference plate and floor surface 113 b at a particular pointcorresponding to the particular pixel may be determined based on thefringe patterns. FIG. 8A shows the fringe pattern formed by plotting theintensity values for a particular pixel from different images using 400wavelengths and FIG. 8B shows the fringe pattern formed by plotting theintensity values for a particular pixel from different images using 21of the wavelengths of FIG. 8A ranging between about 907.5 nm to about911.5 nm.

According to one embodiment, after these spectral interference signals(interferograms) are captured, they may be uniformly resampled in thewavenumber (2π/λ) space (k-space) using existing interpolationtechniques. Fast Fourier Transform (FFT) of the k-space interferogramcan be used to retrieve distance information. Other discrete frequencydomain transforms may also be used. Analytically, this k-space fringepattern may be algebraically expressed using an ABC model as

I(k)=A[1+B·Cos(C)]

where,

-   -   A=(I_(max)+I_(min))/2 is the DC amplitude of the f mge pattern,    -   B=(I_(max)−I_(min))/(I_(max)+I_(min)) is the fringe visibility,    -   and C=2k·d, is the phase factor at each wavelength for distance        d.

According to embodiments, fringe patterns such as shown in FIG. 8A maybe constructed by acquiring intensity data for a particular pixel usingone wavelength at a given time. Therefore, it follows that the timerequired to acquire the interferogram is directly proportional to thenumber of wavelengths used. Since the scanning stage on which the sampleunder measurement is kept is continuously moving, surface sampling sizewill depend on data acquisition time. To improve sample pixel size, itis important to keep data acquisition time to a minimum, as it isundesirable to slow down the scanning stage. In turn, this means fewernumber of fringes per sample size. An example of a limited number offringes is shown in FIG. 8B. The depth profile obtained by Fouriertransforming interference fringes will exhibit an axial resolution thatis in direct correlation to the number of data points in the fringepattern and the number of fringes.

In some other embodiment where a grating spectrometer is used, thedetector size may be such that the spectrometer can output only alimited number of spectral fringes. To address this limitation, oneembodiment is a method of improving axial resolution of interferometricmeasurements where only a limited number of spectral fringe data areavailable. According to one embodiment, the goal of improving the axialresolution of interferometric measurements obtained with only a limitednumber of spectral fringe data may be achieved by extending theacquired, limited number of wavelengths, spectral fringe pattern to alarger wavelength domain by appending the measured data with estimatedspectral data. This estimated spectral data may be generated using theA, B and C coefficients of the ABC model above. The ABC coefficients maybe determined from the measured spectral fringe data and by calculatingthe k-space intensities I(k)s at the added synthetic extensionwavelengths.

The axial resolution of interferometric measurements taken in a mediumwith refractive index n is L_(r)=0.5. λ²/(n·Δλ). Δλ is the spectralbandwidth, with larger bandwidths correlating with higher measured axialresolutions. One exemplary implementation of spectral extensionaccording to an embodiment is illustrated using the short fringe pattern(based upon a limited number of wavelengths) given in FIG. 9A andextended fringe pattern in FIG. 10A. As shown in FIG. 9A, the shortfringe pattern has 21 wavelengths (Δλ=4.15 nm) and as shown in FIG. 10A,the extended fringe pattern has 128 wavelengths (Δλ=26.6 nm, N=128),meaning that in FIG. 10A, estimated interference fringe patternsequivalent to 107 additional wavelengths were appended to the measuredinterference fringe patterns of the 21 original wavelengths of FIG. 9A.

The step of resolving the extended spectrum to retrieve the depthprofile may comprise use of a Fast Fourier Transform (FFT) or HilbertTransform or model-based fringe analysis techniques.

FIG. 9B and FIG. 10B show the power spectral density (PSD) obtained froma FFT of the k-space interferogram obtained from spectral fringes inFIG. 9A and FIG. 10A, respectively. As can be seen from FIGS. 9B and10B, the spectral extension technique described herein enhances theaxial resolution of the interferometer without, however, increasingacquisition times and without increasing the number of wavelengths usedand increasing the consequent data acquisition time or reducing thespeed of the sample stage. Indeed, as shown the PSD shown in FIG. 10B isadvantageously and substantially narrower than the PSD shown in FIG.10B, obtained from the spectral extension technique discussed above.Although the spectra was increased from N=21 in FIG. 9A to N=128 in FIG.10A, the use of N=128 was for illustrative purposes only, as N can beany integer.

An iterative process may be employed to minimize a least-squarecriterion between the estimated interference pattern and the measuredinterference pattern.

In the embodiments presented herein, any error introduced by spectralextension technique will be mitigated in the height or depthmeasurements, since these measurements are obtained by calculating thedifferences in the distances measured between the reference platesurface and the sample surfaces. Moreover, in these and otherembodiments, any error introduced in absolute distance measurements bythe spectral extension technique disclosed and shown therein is a fixedbias error that may be readily canceled out using calibration standards.

The present spectral extension embodiments may be implemented indifferential metrology applications that employ swept sourceinterferometric spectrometry. In High Volume Manufacturing, the waferthroughput (Wph) is directly proportional to the number of wavelengthsλs employed in making the measurements. Fewer λs will speed upmeasurements but lead to reduced axial resolution. With the spectralextension method using ABC model disclosed herein, however, both Wph andenhancements in depth resolution enhancements are achieved.

Terms used in this disclosure and especially in the appended claims(e.g., bodies of the appended claims) are generally intended as “open”terms (e.g., the term “including” should be interpreted as “including,but not limited to,” the term “having” should be interpreted as “havingat least,” the term “includes” should be interpreted as “includes, butis not limited to,” etc.).

Additionally, if a specific number of an introduced claim recitation isintended, such an intent will be explicitly recited in the claim, and inthe absence of such recitation no such intent is present. For example,as an aid to understanding, the following appended claims may containusage of the introductory phrases “at least one” and “one or more” tointroduce claim recitations. However, the use of such phrases should notbe construed to imply that the introduction of a claim recitation by theindefinite articles “a” or “an” limits any particular claim containingsuch introduced claim recitation to embodiments containing only one suchrecitation, even when the same claim includes the introductory phrases“one or more” or “at least one” and indefinite articles such as “a” or“an” (e.g., “a” and/or “an” should be interpreted to mean “at least one”or “one or more”); the same holds true for the use of definite articlesused to introduce claim recitations.

In addition, even if a specific number of an introduced claim recitationis explicitly recited, those skilled in the art will recognize that suchrecitation should be interpreted to mean at least the recited number(e.g., the bare recitation of “two recitations,” without othermodifiers, means at least two recitations, or two or more recitations).Furthermore, in those instances where a convention analogous to “atleast one of A, B, and C, etc.” or “one or more of A, B, and C, etc.” isused, in general such a construction is intended to include A alone, Balone, C alone, A and B together, A and C together, B and C together, orA, B, and C together, etc. For example, the use of the term “and/or” isintended to be construed in this manner.

Further, any disjunctive word or phrase presenting two or morealternative terms, whether in the description of embodiments, claims, ordrawings, should be understood to contemplate the possibilities ofincluding one of the terms, either of the terms, or both terms. Forexample, the phrase “A or B” should be understood to include thepossibilities of “A” or “B” or “A and B.”

All examples and conditional language recited in this disclosure areintended for pedagogical objects to aid the reader in understanding theinvention and the concepts contributed by the inventor to furthering theart and are to be construed as being without limitation to suchspecifically recited examples and conditions. Although embodiments ofthe present disclosure have been described in detail, it should beunderstood that various changes, substitutions, and alterations could bemade hereto without departing from the spirit and scope of the presentdisclosure.

What is claimed is:
 1. An optical system configured to measure a raisedor receded surface feature on a surface of a sample, comprising: abroadband light source; a tunable filter optically coupled to thebroadband light source, the tunable filter being configured to filterbroadband light emitted from the broadband light source and to generatea first light beam at a selected wavelength; a linewidth control elementconfigured to receive the first light beam and to generate a secondlight beam having a predefined linewidth and a coherence length that isa function of a minimum height of the raised or receded surface featureon the sample; collimating optics optically coupled to the second lightbeam and configured to collimate the second light beam; collinearizingoptics optically coupled to the collimating optics and configured toalign the collimated second light beam onto the raised or recededsurface feature of the sample, and a processor system and at least onedigital imager configured to measure a height of the raised or recededsurface from light reflected at least from the raised or recededsurface.
 2. The optical system of claim 1, wherein the broadband lightsource comprises at least one of a white light laser and anultra-broadband source.
 3. The optical system of claim 2, wherein theultra-broadband source comprises one of an arc lamp, a laser drivenplasma source and a super luminescent diode (SLED).
 4. The opticalsystem of claim 1, wherein the tunable filter is configured to generatedifferent wavelengths of light having a finite linewidth at eachwavelength.
 5. The optical system of claim 1, wherein the broadbandlight source and the tunable filter form a tunable light source that isconfigured to generate the first light beam at a plurality of wavelengthsteps and such that the first light beam has a predetermined linewidthat each of the plurality of wavelength steps.
 6. The optical system ofclaim 1, wherein the linewidth control element comprises a plurality ofinterference filters, each of the plurality of interference filtershaving a defined passband.
 7. The optical system of claim 1, wherein thelinewidth control element comprises a grating-based wavelength selectorelement.
 8. The optical system of claim 7, wherein the linewidth controlelement is configured to receive a controlled beam diameter of the firstlight beam.
 9. The optical system of claim 7, further comprising avariable beam expander disposed between the tunable filter and thegrating-based wavelength selector element, the variable beam expanderbeing configured to selectively control a beam diameter of the firstlight beam incident upon the grating-based wavelength linewidth selectorelement.
 10. The optical system of claim 9, wherein the variable beamexpander is configured to tune the coherence length of the second lightbeam for a given pitch and diffraction order of the grating-basedwavelength selector element at any wavelength of the first light beam.11. The optical system of claim 9, further configured to image differingz-height ranges of a plurality of raised or receded surface features ona same or a different sample, through control of wavelengths of thefirst light beam by the tunable filter and through control, by thevariable beam expander, of the beam diameter incident on thegrating-based wavelength selector element.
 12. The optical system ofclaim 1, wherein the tunable filter comprises an acousto-optic tunablefilter (AOTF).
 13. A method of measuring a height of a raised or recededsurface feature on a surface of a sample, the method comprising:emitting a broadband light beam; selectively filtering the broadbandlight beam to generate a first light beam at a selected wavelength;controlling a linewidth of the first light beam to generate a secondlight beam having a predefined linewidth and a coherence length that isa function of a minimum height of the raised or receded surface featureon the sample; collimating the second light beam; collinearizing thecollimated second light beam and directing the collinearized secondlight beam onto the raised or receded surface feature of the sample, andmeasuring the height of the raised or receded surface feature on asurface of a sample based upon light reflected at least from the raisedor receded surface feature.
 14. The method of claim 13, wherein emittingis carried out using at least one of a white light laser and anultra-broadband source.
 15. The method of claim 14, wherein theultra-broadband source comprises one of an arc lamp, a laser drivenplasma source and a super luminescent diode (SLED).
 16. The method ofclaim 13, wherein selectively filtering is carried out using a tunablefilter configured to generate different wavelengths of light having afinite linewidth at each wavelength.
 17. The method of claim 13, whereinemitting and selectively filtering are carried out such that the firstlight beam has a predetermined linewidth at each of a plurality ofwavelength steps.
 18. The method of claim 13, wherein controlling thelinewidth of the first light beam is carried out by passing the firstlight beam selectively through one of a plurality of interferencefilters, each of the plurality of interference filters having a definedpassband.
 19. The method of claim 13, wherein controlling the linewidthof the first light beam is carried out using a grating-based wavelengthselector element.
 20. The method of claim 19, further comprisingselectively controlling a beam diameter of the first light beam incidentupon the grating-based wavelength selector element.